As storage batteries, there are presently being mainly used lead batteries and nickel-cadmium batteries, although silver oxide-zinc batteries are also being used to some extent.
Recently, development of a new storage battery has been expected in relation to the miniaturization and weight reduction of electronic machines and apparatus.
The nickel-cadmium battery now in use usually employs for its positive electrode an active material having a cobalt content of from 2 to 10 wt%. On the other hand, employed as its negative electrode is a sintered nickel substrate having a porosity of about 80%, or a plate prepared by fixing cadmium hydroxide and metallic cadmium as active materials on a nickel-plated perforated steel. When this conventional nickel-cadmium battery is charged, the charging should be carried out so as to apply an electric current in an amount corresponding to 105 to 150% of its charge capacity. This is because oxygen is apt to generate during charge, and this lowers the charge efficiency of the positive electrode, resulting in a low Coulomb efficiency. That is, the conventional nickel-cadmium battery has a drawback that its capacity is recovered only when the charging is performed so as to overcharge it. This means that the conventional nickel-cadmium battery should be overcharged while oxygen is being allowed to evolve from the positive electrode. In the nickel-cadmium battery, the oxygen which has evolved from the positive electrode as a result of the reaction shown by equation (1) is absorbed on the negative plate as shown by equation (2). EQU 4OH.fwdarw.O.sub.2 +2H.sub.2 O+4e (1) EQU O.sub.2 +2H.sub.2 O+4e.fwdarw.4OH (2)
Therefore, even in the overcharge region, hydrogen does not evolve from the negative electrode and the oxygen generated from the positive electrode is absorbed on the negative electrode, resulting only in heat generation. For this reason, the charging of a nickel-cadmium battery is performed with a constant current by means of a method in which the charging is stopped after an increase in temperature of the battery due to the heat generated in the overcharge region is detected with a thermistor or the like, a method in which the charging is stopped after a phenomenon wherein a decrease in charging voltage occurs due to gas absorption is detected, or a method in which the charging is stopped after an increase in inner pressure is detected with a pressure sensor. In order to perform quick charge according to this charging method, the gas-absorbing performance in the overcharge region should be improved. Since the rate constant for this gas absorption is in proportion to the partial pressure of oxygen, a cylindrical battery is preferably subjected to quick charge because a cylindrical battery case is difficult to suffer from deformation or breakage even if the inner pressure of the battery increases. Even in this case, charging at 1C is the limit. On the other hand, .from the viewpoint of energy density rectangular battery is more advantageous than a cylindrical one. However, since the withstand pressure of the rectangular battery case is lower than that of the cylindrical battery case, its withstand pressure is about 5 kg/cm.sup.2 at most even if a metal such as iron, etc. is employed as the material for the case. Hence, charging at 0.3 C is the maximum limit. As a method of charging this nickel-cadmium battery, there may be mentioned a method in which an increase in charging voltage is detected, but the value is 100 to 150 mV at most. Further, since the increase in voltage becomes small as the temperature increases, not only it becomes necessary to make temperature corrections, but also the method is not so reliable. Furthermore, in the case of constant-voltage charging, a so-called runaway phenomenon is produced, leading to breakage of batteries in some cases.
Moreover, the active materials of negative electrode for sealed-type batteries currently in use have a means for preventing the evolution of hydrogen from the negative electrode during charge. If hydrogen gas evolves during charge, the hydrogen remains in the battery as it is, though only an extremely small proportion thereof is absorbed on the positive electrode. As a result, accumulation of hydrogen occurs, leading to lowering the partial pressure of oxygen and, hence, making the reaction for absorption of oxygen gas difficult to take place. In this case, when the inner pressure has reached to a value at which the safety valve operates, not only hydrogen gas but also oxygen gas are released from the valve. This means a decrease in amount of the electrolyte and leads to a decrease in capacity. Further, the evolution of hydrogen gas is undesirable also from the viewpoint of safety.
As a specific means for prevention of the evolution of hydrogen gas, generally employed is a method in which an excess of cadmium hydroxide, called cadmium hydroxide for reserve, is fixed besides an active material incorporated in an amount in proportion to the capacity of the positive electrode.
The amount of the excess cadmium hydroxide should be such that it can compensate at least the following main items:
(a) the amount of the positive active material increased by the formation of nickel hydroxide, which is the same as the active material, resulting from the oxidation of a nickel substrate or the like, which is a support for the active material, through charge and discharge; PA0 (b) the amount of the oxygen which is consumed in the decomposition of a separator after being generated during charge; and PA0 (c) the amount of oxygen in proportion to the product of an allowable inner pressure of the battery and a space volume of the battery. PA0 (a) Since sealed-type batteries mainly employ a sintered nickel substrate for negative electrode, the hydrogen overvoltage becomes low. PA0 (b) There is a problem in charge characteristics of cadmium hydroxide as the negative active material. That is, the charge efficiency of cadmium hydroxide becomes low especially when charging is performed at low temperatures or at high rates, and the time when hydrogen evolves varies. Furthermore, particles of cadmium hydroxide become larger with the progress of charge-discharge cycle, resulting in low charge efficiency. PA0 (c) The discharge capacity of cadmium as the negative active material highly depends upon the discharge rate.
In either case, cadmium hydroxide present in electrode is converted into metallic cadmium in an amount corresponding thereto. The amount of this cadmium hydroxide for reserve varies depending upon the separator material and the shape and use conditions of the battery, but it is generally 40 to 100% of the theoretical capacity of the positive active material. If expressed based on the weight of the positive active material, the content in the negative electrode of cadmium hydroxide including the cadmium hydroxide for reserve is usually from 1.77 to 2.53 times the amount of the positive active material. As described above, conventional batteries should contain an excess of cadmium hydroxide in amounts in proportion to or larger than the amounts of the active materials of the positive electrode. This is not preferable from the viewpoint of battery capacity, but is a necessary means for prevention of the evolution of hydrogen from the negative electrode. This means that the easy method of controlling the charge of the sealed-type lead storage battery only by detecting a rise in charging voltage is not practically applicable to the charge of the nickel-cadmium battery.
Under these circumstances, there has been proposed a method in which a negative-limited cell is prepared by the use of a cadmium negative electrode employing a collector of iron or cadmium, and a change in voltage during charge is detected. This method is reported in 9th Intersociety Energy Conversion Engineering Conference Proceedings, p881 (1974). However, this battery has a drawback that its capacity decreases greatly with a lapse of charge-discharge cycle, and this phenomenon becomes remarkable especially at a temperature as low as 0.degree. C. or as high as 40.degree. C.
On the other hand, in a positive-limited cell employing a positive electrode comprising, as main components of its active material, hydroxides having a cobalt content of 15 to 85 wt% based on the sum of nickel and cobalt and a negative electrode comprising, as main components of its active material, cadmium hydroxide and metallic cadmium, by setting the cadmium hydroxide content in the negative electrode at 0.95 or smaller in terms of the weight ratio of the cadmium hydroxide to the positive active material, that is, by incorporating no cadmium hydroxide for reserve, a method for the control of charge by detecting a voltage increase during the charge, said method being regarded as difficult to be applied to the conventional nickel-cadmium battery can extremely easily be carried out. Further, there is no need for cadmium hydroxide for reserve, as different from conventional batteries. It has been suggested that, because of the above, this battery can be made to have a higher capacity, and not only a cylindrical battery but also rectangular one can be charged at a rate as high as 1C or higher, such a quick charge being almost impossible with conventional batteries (Unexamined Published Japanese Patent Application No. 63-250068).
However, besides the disadvantage of having difficulty in being charged at low temperatures, it has a drawback that the incorporation of cobalt in a large proportion in the active material of the positive electrode result in a high cost. Further, it has another disadvantage that th- control of charge is inferior to that for the lead storage battery, in view of the fact that the lead storage battery shows a rapid rise in the final stage of charge and the difference in voltage is as large as 600 mV.
It has become clear from the above that the method for the control of the constant-voltage charging of lead batteries is difficult to apply to nickel-cadmium batteries, and also a background thereof has become clear. The problems to be solved by the present invention will further be described below in detail with reference to sealed-type batteries as an example.
Like lead, cadmium is known as a metal showing a high hydrogen overvoltage. The control of the charge of the lead battery is generally performed by a method in which a potential change is detected until the potential reaches a value where hydrogen evolves from the negative electrode of lead; whereas in the case of the nickel-cadmium battery, generally employed is not a sealed battery for which the control of charge is done by detecting a potential change until the potential reaches a value where hydrogen evolves from the cadmium electrode, but a method in which a temperature change or potential change resulting from a gas-absorbing reaction on the cadmium electrode is detected. A chief reason for this is that since there is no need for the consideration of self-discharge, i.e., oxidation of cadmium in the negative electrode with evolution of hydrogen, there can be taken a means of allowing the oxygen generated on the positive electrode to be efficiently absorbed on the negative electrode by the use of a pressure-resistant vessel. On the other hand, as other reasons for that the method of taking an advantage of a high hydrogen overvoltage at the negative electrode to detect the potential change, as in the case of the lead battery, is not generally employed, the following may be mentioned.